1. Field of the Invention.
The invention relates controlling the content of magnesium in molten aluminum and, more particularly, to a method and apparatus for injecting halogen gas into the molten aluminum, sensing various characteristics of gases evolved in response to the injection of the halogen gas, and controlling the injection of halogen gas so as to produce evolved gases having desired characteristics (and, concurrently, a desired content of magnesium in the molten aluminum).
2. Description of the Prior Art.
In the production of molten aluminum, it is desirable to control, i.e. reduce or remove, excessive quantities of magnesium. Desirably, the magnesium content of the molten aluminum will be reduced to low levels, for example, less than about 0.2 percent by weight. If the magnesium content of the molten aluminum is excessive, the molten aluminum may be unsuitable for further die cast and foundry ingot production. Unfortunately, recycled aluminum, particularly used beverage containers, often contains a high content of magnesium that must be reduced to low levels before further processing is possible.
An additional problem in the processing of recycled aluminum is the removal of hydrogen. It is desirable to remove hydrogen from molten aluminum because if the hydrogen is not removed before the molten aluminum solidifies into a cast product, gaseous defects in the form of gas holes, blows, and microporosity can result in impaired physical and mechanical properties of the resultant cast aluminum.
Yet an additional concern relates to the removal of alkali metal impurities such as lithium, calcium, and sodium. While these impurities usually are present in small concentrations of less than 100 parts per million, they nevertheless can be quite detrimental. In particular, if the resultant cast aluminum is flat-rolled, the alkali metal impurities can cause cracking and tearing in subsequent fabrication operations.
The usual technique for removing, or reducing, the noted impurities is the injection of a "cleansing gas" into the molten aluminum. A typical cleansing gas includes an inert gas such as nitrogen or argon, together with halogen compounds containing chlorine and/or fluorine. While chlorine is the preferred halogen gas, and while most of the discussion herein will be with respect to chlorine, it is to be understood that the present invention is amenable to use with chlorine, fluorine or any of the other halogen gases, either alone or in combination with other gases. Also, while the term "demagging" is used herein to describe the reduction or removal of magnesium to desired low levels, it is to be understood that such term also includes the reduction or removal of hydrogen and alkali earth metals.
In the demagging of molten aluminum with chlorine, the following primary reactions occur: EQU 2/3 Al+Cl.sub.2 .fwdarw.2/3 AlCl.sub.3 EQU 2/3 AlCl.sub.3 +Mg.fwdarw.MgCl.sub.2 +2/3 Al
When gaseous chlorine is introduced into molten aluminum, gaseous aluminum chloride is formed which further decomposes to react with magnesium that is present. The resultant product, magnesium chloride (MgCl.sub.2), is a liquid phase which is less dense than aluminum and which therefore rises ultimately to the surface in the form of dross, where it may be removed by skimming. Kinetic factors such as rate of mixing and contact area also have an effect on the efficiency of the magnesium removal process. Accordingly, the addition of chlorine by itself does not guarantee effective magnesium removal.
For a stoichiometric reaction, 2.95 pounds of chlorine are required to remove one pound of magnesium. However, if process factors are such that the reaction is not efficient, substantially more than 2.95 pounds of chlorine may be required. Inefficient reactions waste time, consume excessive chlorine, and usually result in substantial emissions and fumes, creating environmental hazards and corrosion problems. Unreacted chlorine, aluminum chloride, and complex oxychlorides react with moisture in the air to create acidified products that corrode most metal structures, even stainless steels. Hence, demagging processes must be both favorable and efficient for secondary smelting and recycling applications.
Heretofore, several techniques have been utilized to demag molten aluminum. Aluminum chloride and fluoride salts have been used individually, and chlorine salt fluxes plus chlorine injection also have been used. While these techniques can produce very good demagging efficiency, reaction times are long and they can produce excessive emissions and also present salt flux disposal problems. The so-called "scrubber bell" process uses chlorine gas injection and captures the resultant emissions under a hood or bell. These emissions subsequently require scrubbing with water before discharge, and consequently the overall treatment and disposal costs can be prohibitive.
The most common technique in use today for demagging molten aluminum is the injection of gaseous chlorine by means of a circulation/gas injection pump. The pump employs an impeller that creates a high velocity molten metal discharge which shears gas being injected through a so-called injection tube. This creates a very wide dispersion of extremely small bubbles which improves the efficiency of the demagging process. The high surface area associated with the very small bubbles results in very high reaction rates between the chlorine, aluminum, and magnesium. Thus, favorable reaction kinetics are achieved, as well as favorable thermodynamics.
Despite the effectiveness of the demagging operation by the use of a circulation/gas injection pump, certain problems have not been addressed. One of these problems relates to controlling the rate of chlorine injection so that adequate chlorine is available for reaction purposes, but excessive chlorine is not injected into the melt. If excessive chlorine is injected into the melt, unreacted chlorine will be contained in the gases being evolved from the molten aluminum. "Unreacted chlorine" as used herein means not only gaseous chlorine per se, but also aluminum chloride (AlCl.sub.3) that has not reacted with magnesium contained in the molten aluminum. The presence of unreacted chlorine in the gases evolved from the molten aluminum is quite undesirable, as noted earlier. Similarly, the term "unreacted halogen gas" as used herein means halogen gas that has not reacted with magnesium contained in the molten aluminum, thereby leading to the emission of undesirable by-products from the molten aluminum.
The most common technique for controlling the rate of chlorine injection is a manual one, where the pump operator observes the evolved gases and increases the pump speed and/or reduces the chlorine supply upon observing a white plume indicative of excessive chlorine consumption.
Another technique for controlling chlorine injection is to take metallurgical samples of the melt and analyze the samples for magnesium content (by atomic absorption or optical emission spectroscopy). A problem with taking metallurgical samples and analyzing them is a significant lag time between chlorine injection and a resultant affect on magnesium content. While the previously described visual observation technique and the metallurgical sample technique sometimes are used in combination, they still represent an "after-the-fact" determination of proper chlorine injection flow rate.
Another technique that has been used to control chlorine injection flow rate is that of an on-line melt sensor (electrode) that senses the magnesium content of the melt. Although a melt sensor is useful to determine the state of the demagging process, it, like the previously described techniques, is an after the fact technique that cannot be used to optimize the rate of chlorine injection as the demagging process is occurring.
Desirably, a technique would be available to control the rate of chlorine injection that would attain maximum demagging efficiency while minimizing the discharge of unreacted chlorine. In effect, it is desired to be able to limit the rate of chlorine injection prior to, or at the onset of, undesirable emissions. It also would be desirable for any such control technique to be usable with a wide variety of furnace configurations and to present minimal difficulties in installing any necessary equipment and for any such installed equipment to be as unobtrusive as possible. Yet an additional concern relates to making the control technique as automatic in operation as possible, so that control decisions by the operator are reduced or eliminated.